The electric motors used for driving downhole mechanisms are very elongated, sometimes of a length of more than one hundred times their diameter. If only one section of electric machine is used, its shaft is as long as necessary to fulfill the given power requirements, which may be 10 m and even longer. There are two negative effects associated with the long shaft; angular twisting of the shaft and deflections of the shaft. Angular twisting is especially problematic in permanent magnet machines where control strategies are based on information of exact position of the magnetic poles relative to the stator winding. The problem aggravates in permanent magnet machines with many poles (eight and more).
U.S. Pat. No. 6,388,353 B1 (Liu et al. (2002)) suggests a solution to the problem of a long twistable shaft by using the permanent magnets of adjacent rotor sections being offset from each other by a predetermined angular displacement, wherein the sum of the predetermined angular displacements between each rotor section is approximately equal to the angular twisting of the shaft under a standard load.
Deflections may be the problem in all machines types requiring support bearings along the shaft or use of sectioned shafts. For example, patent application US 2007096571 A1 (Yuratich (2004)) and US 2008111434 A1 (Head (2008)) present ideas on connection of the sections.
The use of sectioned shafts with rotors and sectioned stators inside the same housing or totally separate machines with connected shafts raises a problem of controlling the sections or the machines. The idea of independent control of each section appears in the six patent applications cited below. In these patent applications often other problems are solved at the same time, for example operation of the stages/sections at different speeds.
Patent application US 2002011337 A1 (Grant (2001)) claims a plurality of inverters adapted to be installed downhole, one for each motor, such that each motor can be controlled separately of the others.
Patent application US 2002066568 A1 (Buchanan et al. (2001)) suggests that each stage of the plurality of stages is independently controllable.
Patent application CA 2,375,308 A1 (Pettigrew (2002)) suggests that motors are mechanically connected to one another and supplied with electrical power independently from one another, where the supply is from the surface.
Already cited patent application US 2007096571 A1 presents the idea of separate supply leads for supplying first and second sections with electrical power from the surface, which is in a way repeating the idea in CA 2,375,308 A1.
Patent application EP 1901417 A1 (Schmitt (2006)) claims separate power supplies and control systems for each motor sub-unit, wherein the control systems are arranged to drive the phases of the motor sub-units together.
Patent application WO 2008148613 A2 (Neuhaus et al. (2008)) also uses a principle of independent control of motor sections. Inverters are disposed directly at each motor section. Switched reluctance drive technology is used.
All the abovementioned patents have downhole applications as the main area of use. The ideas from the above patents relevance to the current application may be summarized in the following set of concepts:                motor is controlled from one source (inverter) and uses skewed magnets to compensate for the angular shaft twisting;        motor is controlled from one source (inverter) and consists of several sections to avoid rotor/shaft deflections;        motor has two sets of windings, each fed separately from the surface or several sections each fed separately from the surface;        a certain tool has several sections, each comprising a motor together with some other mechanism, where each section is controlled independently from the others by a separate inverter; and        motor has several sections, each controlled independently from a separate inverter located either at the section or in a separate unit installed near the motors.        
The set of ideas presented in the current patent is different from any of the listed concepts.